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<title>ROM</title>
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<h1><img align="center" height="32" width="32" src="../../../icons/rom.gif">
<em>ROM</em></h1>

<p><table>
<tr><td><strong>Library:</strong></td>
	<td><a href="index.html">Memory</a></td></tr>
<tr><td><strong>Introduced:</strong></td>
	<td>2.1.0</td></tr>
<tr><td valign="top"><strong>Appearance:</strong></td>
	<td valign="top"><img src="../../../img-libs/rom.png"></td></tr>
</table></p>

<h2>Behavior</h2>

<p>The ROM component stores up to 16,777,216 values (specified in the Address
Bit Width attribute), each of which can include up to to 32 bits
(specified in the Data Bit Width attribute). A circuit can access
the current values in ROM, but it cannot change them. The user can modify individual values
interactively via the Poke Tool, or the user can modify the entire
contents via the Menu Tool.</p>

<p>Unlike the RAM component, the ROM component's current contents are
stored as an attribute of the component. Thus, if a circuit containing
a ROM component is used twice, then both ROM components will hold the
same values. Also because of this behavior, the current ROM contents are
stored in files created by Logisim.</p>

<p>Current values are displayed in the component. Addresses displayed
are listed in gray to the left of the display area. Inside, each value
is listed using hexadecimal. The value at the currently selected address
will be displayed in inverse text (white on black).</p>

<h2>Pins</h2>

<dl>

<dt><var>A</var> on west edge (input, bit width matches Address Bit Width attribute)</dt>
<dd>Selects which of the values are currently being accessed by the
circuit.</dd>

<dt><var>D</var> on east edge (input/output, bit width matches Data Bit Width attribute)</dt>
<dd>Outputs the value at the currently selected address at the
<var>D</var> pin if <var>sel</var> is 1 or floating. If <var>sel</var>
is 0, then <var>D</var> will be floating.</dd>

<dt><var>sel</var> on south edge (input, bit width 1)</dt>
<dd>If you have just one ROM module, ignore this input.
If you have multiple ROM modules in parallel, you can use this input
to enable or disable the entire ROM module, based on whether the value is
1 or 0. In other words, when this is 0, no value is emitted on
the <var>D</var> output.</dd>

</dl>

<h2>Attributes</h2>

<p>When the component is selected or being added,
the digits '0' through '9' alter its <q>Address Bit Width</q> attribute
and Alt-0 through Alt-9 alter its <q>Data Bit Width</q> attribute.</p>

<dl>

<dt>Address Bit Width</dt>
<dd>The bit width of the address bits. The number of values stored in
ROM is 2<sup><var>addrBitWidth</var></sup>.</dd>

<dt>Data Bit Width</dt>
<dd>The bit width of each individual value in memory.</dd>

<dt>Contents</dt>
<dd>Stores the contents of memory.</dd>

</dl>

<h2>Poke Tool Behavior</h2>

<p>See <a href="../../guide/mem/poke.html">poking memory</a>
in the <em>User's Guide</em>.</p>

<h2>Text Tool Behavior</h2>

<p>None.</p>

<h2>Menu Tool Behavior</h2>

<p>See <a href="../../guide/mem/menu.html">pop-up menus and files</a>
in the <em>User's Guide</em>.</p>

<p><a href="../index.html">Back to <em>Library Reference</em></a></p>

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